The invention concerns a device to make polyurethane articles, covered with a decorative film, which consists of a molding apparatus equipped with means able to perform the function of a perimeter undercut of the article.
The invention also concerns a method to produce polyurethane articles, wherein the steps of thermoforming the covering film and injecting/molding the polyurethane material are carried out in a single piece of apparatus and substantially in a single step, allowing to eliminate the subsequent, laborious operations of filling, sanding and painting in order to obtain the finished product.
The invention also allows to accelerate and automate both the steps of loading the film into the mold and also those of moving and discharging the covered polyurethane articles at the end of the forming/molding process.
The invention also concerns the polyurethane articles, covered with a decorative film, obtained with this method and this device. The invention is applied in the field of molding polyurethane articles, such as small articles of furniture, furniture wings, panels for automatic distributing machines or domestic appliances, containers, etc.
The invention reduces to a minimum the number of working stages needed to complete the production process, reducing the relative times and spaces, and allows to obtain finished products of high quality and surface finish, with substantially any desired shape.
There are various methods, in the state of the art, to produce polyurethane articles, which are finding more and more varied applications in many fields of use.
Such articles are obtained, at present, essentially by means of two production processes.
A first process provides to paint the surface of the article after it has been removed from the mold. This process requires complex operations on the article which has been removed from the mold, such as trimming, filling, sanding and painting. These operations are not only very laborious, but also they do not give satisfactory results which can be repeated with acceptable levels of standardisation. Moreover, they require considerable manpower and long production times.
Furthermore, these operations considerably limit the possibility of automating the production process.
A second process provides to apply a covering, protective film at least on the visible face of the polyurethane article.
In this case, the state of the art provides to thermoform a covering film in a first mold, to insert it into another mold, and then to inject the polyurethane inside this second mold. By means of this film, the polyurethane articles are ensured the necessary surface resistance and the desired aesthetic quality.
However, this technique, using pre-formed films, also has some disadvantages, including the surface quality of the product, which is never regular due to the plays in elasticity of the products which are heated at different times, and to the adaptation of the thermoformed film to the mold because of the injection of the polyurethane. A further problem is the absence of a constant quality.
Moreover, this technique requires large work surfaces, both to move the material and also to perform the various steps described above.
The present Applicant, in the patent application PCT/IBOO/01084, proposed an innovative method to produce covered polyurethane articles, by means of thermoforming the covering film in a mold and injecting polyurethane material into the same mold, wherein most of the disadvantages listed above are eliminated.
The articles obtained with the method described above emerge from the mold substantially finished, and only require the perimeter trimming of the film.
Document WO 00/12282 describes a method to mold objects made of plastic material wherein a covering sheet is made to adhere, by means of jets of compressed air, against the wall of the upper part of the mold, and then the plastic material is injected from below in order to form the covered article.
Document WO 96/03267 describes a method to produce covered thermoplastic articles wherein a covering film is arranged inside the cavity of the mold, and then the thermoplastic material is injected to thrust the film against the wall of the mold. This method provides that the edges of the film can be released during the injection process to allow the film to move into contact with the wall of the mold while the injected material fills the cavity thereof.
Document DE-A-4033297 shows a method to produce covered thermoplastic articles substantially identical to the previous ones.
In these prior art documents a first problem concerns the correct finishing of the visible surface of the covered polyurethane article, which must not have any visible signs or marks deriving from the forming and injection process.
Another problem concerns the possibility of moving and discharging, in an easy and rapid fashion, the finished covered article at the end of the molding process.
A further problem concerns the possibility of achieving covered polyurethane articles with particular shapes and contours, for example with undercuts which may be different from each other on each occasion.
The Applicant has devised and embodied this invention to solve these and other problems, and to obtain further advantages as will be shown hereafter.
The invention is set forth and characterized in the respective main claims, while the dependent claims describe other characteristics of the main embodiment.
The purpose of the invention is to achieve a method and a device suitable to supply, in a single molding step and with a single apparatus, a substantially finished polyurethane article characterized by high surface quality, even when there are perimeter undercuts present.
Another purpose is to achieve a molding device equipped with means able to cooperate with the perimeter edge of the article when the mold is closed, such means allowing to form a perimeter undercut and preventing the creation of an imperfect and/or damaged edge in correspondence with the discontinuity created between the coupled surfaces of the male and female parts of said mold.
A further purpose is to achieve a production method wherein the steps can be largely automated and allow the automatic loading of the film in correspondence with the mold, and also the molding, removal and discharge of substantially finished articles, which require only a perimeter trimming of the covering film present.
Another purpose of the invention is to achieve a device wherein the shape, and in particular the perimeter edge, of the article can be changed quickly, without requiring complex operations to dis-assemble and re-assemble parts, or the particular use of the labor force.
The molding device according to the invention comprises a molding apparatus consisting, in a substantially conventional fashion, of a female part, or matrix, suitable for the conformation of the visible face of the article, and a male part, or punch, suitable for the conformation of the non-visible face.
The male part is associated with press means able to move it from an open position to a closed position and incorporates the means to inject the polyurethane material when the mold is closed. The male and female parts of the mold have heating means which can be activated on command, for example, but not only, consisting of heated water circulating in the inner volume of the two elements, or of electric resistances, radiating elements, hot air, or other means.
In one embodiment of the invention, the heating means of the male and female part of the mold are autonomous and independent with respect to each other.
According to one characteristic of the invention, in association with the perimeter edge of the female part of the mold there are means able to perform a function of continuity and connection in correspondence with the zone of discontinuity between the male and female parts of the mold.
Said means are removable and replaceable and comprise, in a preferential embodiment, an annular ring-nut or frame which extends outside, and partly overlapping, the perimeter of the female part, so as to constitute substantially an auxiliary element, or counter-matrix, of the female part.
The annular ring-nut has at least one perimeter edge protruding forwards, in the direction of the molding cavity, which is shaped so as to cooperate with said discontinuity between the two parts of the mold, so that it is possible to form a perimeter undercut.
In this way the annular ring-nut allows to form a perimeter edge of the article without imperfections or irregularities, so that the only operative step required after the article has been removed from the mold is the perimeter trimming of the possible excess film.
According to another characteristic of the invention, the annular ring-nut is associated with movement means which allow it to make at least the following displacements:
to be positioned in a position outside the molding apparatus in order to lay, cut and clamp, on part of its perimeter, a segment of the covering film,
to be translated towards the molding apparatus in order to insert the film into the mold,
to be lowered into substantial co-operation with the perimeter edge of the female part of the mold,
to be raised with respect to the female part, once the forming and molding step is completed, taking the molded article with it, and finally
to be translated laterally with respect to the molding apparatus, in order to allow the article to be removed and discharged, exploiting the excess parts of the film protruding from the figure of the article.
In order to perform the operation of loading the film into the mold, the annular ring-nut includes first vacuum means able to retain the film at least temporarily against the upper surface of the ring-nut.
In one embodiment of the invention, the annular ring-nut also has second vacuum means able to locally create a vacuum in order to achieve the thermoforming of the covering film, said means including external suction holes and slits outside the figure of the molded article.
In a further embodiment, the annular ring-nut has heating means, which can be selectively activated during the forming step of the film.
The heating means, according to a variant of the invention, are autonomous and independent with respect to the heating means of the components of the mold, so as to allow a selectively differentiated heating according to specific requirements.
In a preferential embodiment of the invention, the molding apparatus comprises pressure means, adjustable in pressure, suitable to cooperate with the surface of said annular ring-nut so as to define a sealing outer perimeter which retains the film mechanically during its thermoforming step.
According to a variant, said pressure means and said annular ring-nut have respective means, mating and cooperating, which prevent the formation of wrinkles and creases in the film during the thermoforming step and the subsequent injection of the polyurethane.
In another variant, in the case when the molding apparatus is of the type suitable for the simultaneous formation of two or three figures, there are cutting means provided suitable to cut the film in correspondence with a zone of separation between the two figures, in order to allow the two articles to be removed separately.
In a preferential embodiment, the annular ring-nut is of the interchangeable type, to allow to define perimeter edges, and the relative undercuts, of a different shape according to the specific application.
According to another variant, the molding apparatus comprises auxiliary heating means able to locally determine different temperatures with respect to the rest of the figure.
According to a further variant, the molding apparatus comprises means to deliver compressed air, able to be selectively activated, to collaborate in the operation of forming the film and its adhesion to the inner surface of the female part of the mold.
There are many advantages to the invention, apart from the fact that it obtains a covered article, finished in every part, in a single step.
The complete thermoforming and definitive union of the film and the polyurethane are obtained using heat and pressure generated by the polymerization reaction of the polyurethane itself. This ensures that the quality is constant, that the whole visible surface of the article is perfect and that the results can be repeated; this cannot be obtained with known techniques.
The method eliminates all those steps subsequent to molding, such as filling, sanding and painting, thus bringing considerable advantages in terms of economy, safety (for example, the emission of solvents, waste disposal, fire risks, etc.) and ecology. Moreover, by eliminating these steps it is possible to automate a large part of the operations of the whole production process.
Using a removable and interchangeable ring-nut allows to achieve the desired perimeter undercuts in the article, and therefore allows, substantially in a single step, to produce articles with a complex and elaborate shape.
Moreover, thanks to the removable ring-nut, it is possible to automate the steps to load the film into the mold and to discharge the finished polyurethane article.
In order to obtain higher productivity, a further embodiment provides several molds in line, operating in sequence and having the same characteristics as those described above.